1. Field of the Invention
The present invention relates to an optical encoder device for a small-sized motor which includes a code wheel attached to a motor shaft and disposed between a light-emitting element and a light-receiving element and which optically detects rotation of the motor shaft, and to a method of producing the optical encoder device.
2. Description of the Related Art
There is demand for attaching, to a small-sized motor, a device for detecting the rotational speed and position of the motor. Known detecting devices of this kind perform magnetic detection by use of a magnet and a Hall element, mechanically turn on/off electrical continuity between two brushes, or perform optical detection by use of a photodiode (light-emitting element) and a phototransistor (light-receiving element). The present invention relates to an optical encoder device of this type. A small-sized motor having such an optical encoder device can be used in OA apparatus which requires rotational control, such as a printer.
FIGS. 11A and 11B illustrate a first conventional technique of attaching an optical encoder to a motor (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-357457). FIG. 11A shows a small-sized motor including an integrally assembled a board unit, and FIG. 11B shows the board unit alone. A printed circuit board, which partially constitutes the board unit, has a U-shaped cutout. The board unit having the cutout is moved laterally for attachment to the motor after a code wheel is attached to a motor shaft. In a completed state shown in FIG. 11A, the code wheel having an optical modulation track is disposed in a gap of a photosensor module, which is formed by opposed light-emitting and right-receiving elements, so as to obtain a signal indicating rotation of the motor shaft. The illustrated photosensor module is configured such that light-emitting and light-receiving elements are disposed to face each other and are integrated together, as in the case of a second conventional technique (see FIG. 12B), which will be described later.
Since the code wheel must be disposed in the gap of the integrally formed photosensor module, the code wheel is first attached to an end portion of the motor shaft, and a board unit carrying the photosensor module as shown in FIG. 11B is then moved, positioned, and fixed from a direction perpendicular to the motor shaft such that a mating portion of the motor enters the cutout of the board unit.
As described above, the illustrated structure enables the board unit to be moved, positioned, and fixed from a direction perpendicular to the motor shaft. Therefore, the photosensor module can be attached and fixed to a predetermined position, without damaging the code wheel during the attachment of the photosensor module, which damage would otherwise occur upon contact of the photosensor module with the code wheel.
However, when a cutout is provided on the printed circuit board, the cutout becomes a dead space. Therefore, the degree of integration of patterns formed on the printed circuit board cannot be increased, and thus, the size of the board must be increased.
FIGS. 12A to 12D illustrate a second conventional technique of attaching an optical encoder to a motor. FIG. 12A is a perspective view of a small-sized motor to which a board unit is attached. FIG. 12B shows a photosensor module only, and FIG. 12C shows a code wheel alone. FIG. 12D is a perspective view of the small-sized motor in a state where assembly of the optical encoder is completed. As shown in FIG. 12B, the conventional photosensor module is configured such that light-emitting and right-receiving elements are disposed to face each other and are integrated together, and terminals extending from these elements are fixed to the board unit by means of soldering. The remaining elements, such as a connector, are fixed to the board unit to thereby complete the board unit as an independent unit.
In assembly, as shown in FIG. 12A, the board unit is first attached to the motor. The board unit has holes corresponding to a mating portion and motor terminals provided on the motor. The board unit can be attached to the motor along the axial direction of the motor shaft. A code-wheel-fixing hub is secured to the motor shaft. Subsequently, a code wheel as shown in FIG. 12C is positioned laterally on the hub such that a peripheral portion of the code wheel is inserted into a gap of the photosensor module, and bonded to the hub by means of double-sided tape.
Since the code wheel is inserted laterally while being bended as described above, the code wheel must be made of a transparent material having elasticity, such as polyethylene terephthalate (PET) resin film. When the code wheel is formed of such a transparent material having elasticity, an expensive special film on which an optical modulation track can be formed must be used. In addition, due to dimensional restriction at the time of insertion of the code wheel into the gap of the photosensor module while bending the code wheel, the code wheel and the hub (via which the code wheel is fixed to the motor shaft) must be formed as separate members, and double-sided tape must be used so as to join them together. As described above, the structures according to the conventional techniques increase product cost considerably, because of cost of various components and a large number of assembly steps.